How did Rome expand its territory and maintain control over such a vast area of land?

Rome, especially during the time of the Republic, made sure that their presence was known in the new territories they conquered. They incorporated their ideas on religion, way of life, and culture on the peoples that inhabited their newly-conquered lands. Additionally, the Romans sent senators, praetors, quaestors, and other branches of the legislature in order to ensure that the 'savages' of the land were behaving how the Romans wanted them to behave. Especially unruly provinces (like certain parts of Britain) were plagued with hundreds of Roman soldiers to intimidate them into submission.

the answer to number 5 is fort sumter